Christian Boddum wrote:
For instance we don't do koshinage on our team ,not saying it can't be done , but
if the do it wrong they can easily hurt their backs ,since they are not strong and "stiff" enough in their bodies.
Judo is commonly taught to children, both in the US in Japan (as young as four years old). In Judo koshinage is taught from the very beginning. In spite of hundreds of thousands of young children doing Judo koshinage each year, I am not aware of any especially adverse injury rates. Mainly, the problem with koshinage in Aikido is that people don't do it very much (in general), and because of that it tends to be both performed and taught poorly.
The only thing that I'd really be careful with is over-application of wrist and joint techniques.